As illustrated in FIG. 7, a wireless communication system Sa for mobile phones is constituted of an IP network that includes a switching center 300a, such as a Mobility Management Entity (MME)/Serving Gateway (S-GW), that is connected to an Internet Protocol (IP) network 400a and multiple base stations (eNB: evolved Node B) 200a that are connected to the switching center 300a. 
With each wireless communication apparatus (hereinafter, referred to as a “user terminal (UE)”), such as mobile phones, data transmission is performed with another user terminal 100a via the nearest base station 200a. 
A communication specification called Long Term Evolution (LTE) is actively studied as a new standard used in such a wireless communication system Sa. LTE has been drawing attention in the 3rd Generation Partnership Project (3GPP) that is one of a communication standardization projects. For example, improvement of layer 2 corresponding to data link layer is enhanced. (see, for example, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2” 3GPP, TS36.300, http://www.3gpp.org/ftp/Specs/archive/36series/36.300/36300-850.zip).
As illustrated in FIG. 8, layer 2 of LTE has three sublayers, i.e., a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, and a Medium Access Control (MAC) layer. A user terminal 100 and a base station 200 each have Layer 2.
A PDCP entity and an RLC entity belonging to the PDCP layer and the RLC layer, respectively, are present in accordance with the number of logical channels (LCH) (n in FIG. 8) that are used for radio communication, and each of them are associated with each other.
In each of the n PDCP entities, a header of the PDCP layer is added to transmission data, and then a Protocol Data Unit (PDU) of the obtained PDCP layer is output to the associated RLC entity.
This PDU becomes, in the RLC layer, a Service Data Unit (SDU). The PDU of the RLC layer is obtained by adding a header of the RLC layer by each RLC entity.
In other words, if a PDU of an upper sublayer is output to a lower sublayer, it is treated as a SDU of the lower sublayer. Then, if a header for each sublayer is added to the SDU in the lower sublayer, a PDU of the lower sublayer is obtained.
If a PDU of the RLC layer (hereinafter, referred to as an “RLC-PDU”) is output from each RLC entity to the MAC layer, each of the RLC-PDUs is multiplexed. Then, by adding a header of the MAC layer, the multiplexed RLC-PDU becomes a PDU of the MAC layer (hereinafter, referred to as a “MAC-PDU”) and is subjected to a process of layer 1 corresponding to the physical layer.
At this time, by using a bandwidth that can be used for data transmission or a radio resource, such as electrical power, an MAC entity belonging to the MAC layer determines free space for the MAC-PDU, appropriately allocates the RLC-PDU that is output from each of the n RLC entities to free space of the MAC-PDU, and performs multiplexing.
For example, as illustrated in FIG. 9, in the MAC layer, an RLC-PDU that is obtained by adding an RLC header to an SDU in a first RLC entity (hereinafter, referred to as an “RLC #1”) and an RLC-PDU that is obtained by adding an RLC header to an SDU in a second RLC entity (hereinafter, referred to as an “RLC #2”) are multiplexed by being treated as an MAC-SDU.
A header (MAC header) for the MAC layer or control data is added to each of the two multiplexed MAC-SDUs (user data), thereby the MAC-SDUs each obtain the MAC-PDU (transmission data). Then, the obtained transmission data is transmitted after a process of layer 1 (not illustrated) is performed thereon. Specifically, this transmission data is transmitted between the user terminal 100a and the base station 200a each other.
Furthermore, in the MAC layer, re-transmission control according to a hybrid automatic repeat request (HARQ) using a Stop-and-Wait method consisted of n channels is also executed.
In the HARQ operation in the MAC layer, transmission data is held at the time of transmission, an error correction process and a process for coding a cyclic redundancy check (CRC) is performed on the transmission data.
Then, if a result of receiving the transmission data indicates a reception error (i.e., an error detection result of the CRC coding indicates negative), a receiving end replies a negative acknowledgment (NACK) indicating that status to a transmission end. In contrast, if reception is acceptable (i.e., an error detection result of the CRC code indicates positive), the receiving end replies an ACK indicating that status to the transmitting side.
Thereafter, if the MAC layer of the transmission end receives an NACK, the MAC layer re-transmits the transmission data that is held at the time of initial transmission. If the MAC layer of the transmission end receives an ACK, the MAC layer cancels the MAC-PDU that is held at the time of initial transmission and transmits the subsequent MAC-PDU.
Then, as illustrated in FIG. 10, with the user terminal 100a and the base station 200a having the above-described layer 2, two-way transmission is performed on transmission data between each of the MAC layers of the base station 200a and the user terminal 100a. 
As illustrated in FIG. 10, the base station 200a and the user terminal 100a each include a PDCP layer, an RLC layer, and an MAC layer belonging to layer 2. Furthermore, each of the PDCP layer and the RLC layer includes buffers that temporarily store therein various data received from upstream of the transmission path for the transmission data.
As illustrated FIG. 11, with such a communication system Sa, for example, if an application 500a having low throughput is connected to the user terminal 100a (see (1) of FIG. 11), transmission of the transmission data from the PDCP layer of the user terminal 100a to the application 500a is performed at a lower transfer rate than that of the transmission data from the base station 200a to the user terminal 100a (see (2) of FIG. 11).
Accordingly, at a receiving unit in the user terminal 100a, an overflow of transmission data occurs in the buffer in the PDCP layer (3), thereby there may be a case in which the transmission data cannot be transmitted to the application.
In such a case, the PDCP layer of the user terminal 100a notifies a higher-level transmission data control unit (e.g., a radio resource control (RRC)) 600a of that status. Then, by transmitting a transmission stop request for the transmission data from the RRC 600a to the MAC layer of the base station 200a (see (4) of FIG. 11), transmission of the transmission data from the base station 200a is stopped (see (5) of FIG. 11), thus eliminating the overflow of the buffer (see, for example, Japanese Laid-open Patent Publication No. 2006-157912 and Japanese Laid-open Patent Publication No. 2005-244505).
However, with the conventional communication system Sa, if the base station 200a receives, from the user terminal 100a, the transmission stop request of the transmission data, the base station 200a stops the transmission of the transmission data to the user terminal 100a; however, in such a case, the user terminal 100a cannot receive, from the base station 200a, user data contained in the transmission data nor control data. This control data contains an RLC-control PDU (ACK, NACK, etc.).
Accordingly, because the user terminal 100a cannot receive an ACK nor an NACK associated with data that is transmitted from the user terminal 100a to the base station 200a, the user terminal 100a cannot confirm the delivery of the data that is transmitted from the user terminal 100a. Therefore, as illustrated in (6) of FIG. 11, the RLC layer cannot transmit the transmission data to the base station 200a. 
In this way, a case in which the transmission data cannot be transmitted is not limited to occurring in the user terminal 100a; it may also occur in the base station 200a. For example, if a throughput of an IP network 400a that is connected to the base station 200a via the switching center 300a is relatively low, in a similar manner as in the user terminal 100a described above, an overflow of transmission data occurs in the buffer in the PDCP layer of the base station 200a, thereby there may be a case in which transmission of the transmission data stops between the base station 200a and the user terminal 100a. 